Magnetostrictive elements have such a property that the element has its magnetic permeability varied and is deformed by the variation of the magnetic permeability when subjected to a magnetic field by passing current through a coil disposed in the vicinity of the element. The magnetostrictive element is used as an actuator or oscillator since the amount of deformation and the stress produced due to the deformation are great.
Conversely when the magnetostrictive element is deformed by applying an external force thereto, the element has its magnetic permeability altered by the deformation. For example, JP-U No. 1-105834(1989) discloses a load sensor for detecting the magnitude of an external force by utilizing these characteristics. The sensor comprises a magnetostrictive element, two primary coils provided around the element, and a secondary coil magnetically coupled to one of the primary coils by the magnetostrictive element to provide a differential transformer for detecting a load acting on the magnetostrictive element.
The load sensor is seviceable as a high-sensitivity sensor since great variations in electrical impedance are available in the load detecting range owing to the characteristics of the magnetostrictive element.
Although adapted to realize high sensitivity, such a load sensor has the problem of being great in hysteresis because the magnetostrictive element itself has great energy of crystal magnetic anisotropy to thereby impair the accuracy of detection.
Especially when the load sensor is to be used for washing machines, there is a need to measure the combined weight of laundry weighing hundreds of grams to several kilograms and water of tens of kilograms. Accordingly, the sensor must have a broad load detecting range, whereas the sensor has the problem that the influence of hysteresis markedly appears in the range of small loads to be measured, consequently making it impossible to obtain sufficiently high accuracy of detection.
An object of the present invention is to provide a load detecting system comprising a magnetostrictive element which is reduced in the influence of hysteresis thereof to accurately detect the load acting thereon, and an automatic washing machine having such a highly accurate load detecting system.